Project Summary This is an exploratory R21 proposal to lay the groundwork for a HIV harm-reduction intervention for substance-using MSM who attend circuit parties (CP) in Bangkok, where 1 in 3 MSM are HIV-infected. The study will incorporate a newly developed framework (Modified Social Ecological Model) to contextualize the multi-level risk contexts of circuit parties in Asia. Using this framework, we will (1) conduct formative research to characterize the multi-level risk contexts and identify factors related to HIV risk among MSM attending circuit parties in Bangkok. (2) Using a portal survey methodology, we will quantitatively examine specific longitudinal pathways associated with HIV risks. Specifically, we will (a) estimate the prevalence and patterns of CP attendees who use geo-social applications (apps) to find sexual partners surrounding CP and HIV risk factors associated with these patterns. (b) Examine the influence of app-using patterns on intention, norms and subsequent situational drug use and sexual risk behaviors at CP hosting communities and onward. (3) Assess the feasibility and acceptability of onsite HIV testing and determine HIV prevalence and estimate incidence among MSM attending CPs using a new HIV rapid diagnostic antibody/antigen-based test. Implications for this study are 3-fold: 1) To elucidate pivotal opportunities where efficacious harm-reductions can be applied, e.g. social marketing targeting the social environment of apps, 2) To assess the feasibility of maintaining a volatile substance using MSM cohort via a qualitatively informed approach and 3) To use a new diagnostic technology to identify acute HIV infection and estimate HIV incidence among volatile, substance-using MSM in resource- limited settings. Bangkok is experiencing an explosive HIV epidemic in MSM, despite being a gay-paradise. To devise time-sensitive and culture-specific interventions, we need to understand the social-ecological contexts of newly emerging risk environments. The proposed study will lay the groundwork for a future R01 to develop and evaluate HIV harm-reduction intervention for substance-using MSM who attend CP in Bangkok.